As the inquiry into the atrocity gathered pace, Prime Minister Theresa May revealed the attacker was once linked to violent extremism. He was thought to have been inspired by Islamist ideology, she said.

In an address to the House of Commons -- reconvened less than a day after it was placed in lockdown as the attack unfolded outside -- May vowed that Britain's freedoms and values would remain undiminished.

Four people died in the attack. An American man and a British woman of Spanish origin were killed when Masood rammed a rental car onto the sidewalk on Westminster Bridge.

Masood then stabbed an unarmed police officer in as he stood guard at Carriage Gates, an entrance into a cobbled courtyard frequently used by Members of Parliament and staff. A fourth victim died Thursday night after his life support was withdrawn at a hospital.

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An ISIS-affiliated news agency claimed that the extremist group was behind Wednesday's outrage, which left 29 people requiring hospital treatment. Six people remain critically ill.

Latest developments:

• Khalid Masood had previous convictions for violence but not terrorism offenses, police said.• Kurt Cochran, from Utah, is named by family and his church as having died in the London attacks.• Injured include 10 nationalities besides the British, among them French, South Korean and American.• London Mayor Sadiq Khan led tributes at a vigil at Trafalgar Square, near the scene of the attack.• Addresses were searched in Birmingham and elsewhere and a number of arrests were made.• Queen Elizabeth II voices her "thoughts, prayers, and deepest sympathy" for all those affected.

May: 'Our values will prevail'

In a defiant speech, May vowed that Britain would not be cowed by the attack.

"Yesterday an act of terrorism tried to silence our democracy. But today we meet as normal -- as generations have done before us, and as future generations will continue to do -- to deliver a simple message: we are not afraid. And our resolve will never waver in the face of terrorism.

"And we meet here, in the oldest of all Parliaments, because we know that democracy -- and the values it entails -- will always prevail."

She said the police officer who was killed in the attack, PC Keith Palmer, was "every inch a hero and his actions will never be forgotten."

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May said the current threat level for Britain -- which has been at severe, the second highest, for some time -- would not be raised to critical because there was no specific intelligence that an attack was imminent.

Since 2013, police, security and intelligence agencies have successfully disrupted 13 separate terrorist plots in Britain, she said. "We know the threat from Islamist terrorism is very real. But while the public should remain utterly vigilant they should not -- and will not -- be cowed by this threat."

A tweet from ISIS-affiliated news agency Amaq said the attacker was "a soldier" of ISIS, inspired by its message. However, ISIS has provided no evidence for any direct links to the assailant, whom it did not name.

CNN Terrorism Analyst Paul Cruickshank said the language used by ISIS asserting the attack was by one of its "soldiers" did not necessarily mean the group was claiming direct connections to the attacker. This phrasing has been used in the past by the group for attacks ISIS believes it helped inspire, he said.

Investigation gathers pace

Masood, who was born in Kent and had been living in the West Midlands region of central England, had previous convictions for violence but not for terrorism offenses, police said. His most recent conviction was in 2003 for possession of a knife.

But he was not the subject of any recent investigations and there was no intelligence about his intent to mount an attack, the Metropolitan Police said. May said he was not part of the "current intelligence picture."

Britain's most senior counterterror police officer, Mark Rowley, earlier said inquiries were continuing in London, the West Midlands city of Birmingham and elsewhere. Arrests were made in Birmingham and London.

"It is still our belief -- which continues to be borne out by our investigation -- that this attacker acted alone yesterday and was inspired by international terrorism," Rowley said.

The car rental company whose vehicle was used in the attack said it was cooperating with the police.

Photos:Deadly attack outside UK Parliament

A car is seen crashed into a fence outside the Parliament building in London on Wednesday, March 22. Police have launched a "full counter-terrorism investigation" after an attacker rammed a car into crowds of people and stabbed a police officer on Parliament grounds.

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Photos:Deadly attack outside UK Parliament

An armed police officer stands over the attacker, identified as Khalid Masood, outside Parliament. "It appeared that a car was coming towards the House of Commons mowing down pedestrians on the way," Member of Parliament Gerald Howarth told CNN. "The driver then got access to the parliamentary estate, stabbed a police officer and was shot."

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Photos:Deadly attack outside UK Parliament

Masood is treated by emergency services as police look on at the scene outside the Houses of Parliament. The Metropolitan Police say he was born in Kent, but is believed to have been living in the West Midlands recently. Police say Masood was also known by a number of aliases. "Masood was not the subject of any current investigations and there was no prior intelligence about his intent to mount a terrorist attack," Met Police said in a statement.

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Photos:Deadly attack outside UK Parliament

Member of Parliament Tobias Ellwood, in the glasses, tends to one of the injured people amid the chaos. The man the politician was trying to save was a police officer who died, a witness on the scene told CNN. Authorities identified the deceased officer as Keith Palmer, 48.

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Photos:Deadly attack outside UK Parliament

Emergency workers attend to injured people at the scene.

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Photos:Deadly attack outside UK Parliament

Medics treat a victim on Westminster Bridge.

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Photos:Deadly attack outside UK Parliament

People leave the Parliament building after the incident. Lawmakers were forced to remain in the main debating chamber of the House of Commons as police responded to the incident outside.

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Photos:Deadly attack outside UK Parliament

A person is treated at the scene.

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Photos:Deadly attack outside UK Parliament

Medical aid is provided outside Parliament.

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Tourists were trapped for a time in cars on the London Eye Ferris wheel, which was stopped in the immediate aftermath of the attack.

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Photos:Deadly attack outside UK Parliament

A police officer stands guard near Westminster Bridge and the Houses of Parliament.

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Photos:Deadly attack outside UK Parliament

A person receives medical treatment at the scene.

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Photos:Deadly attack outside UK Parliament

At right, the car of British Prime Minister Theresa May is driven away from Parliament.

London Mayor Sadiq Khan told CNN that the city was protected by the best police and security services in the world. "I'm confident that we are doing all that we can to keep our city safe, to keep Londoners safe and to keep visitors safe," he said. Extra police officers will be patrolling London, he said.

Victims named

Police officer Keith Palmer was killed in the London attack, March 22, 2017.

The dead police officer was named as Keith Palmer, a member of the Metropolitan Police's parliamentary and diplomatic protection command unit. He was a husband and father, and had served in the Met for 15 years.

Aysha Frade, a 43-year-old teacher killed in the attack, was a British national whose mother is a Spanish national, a spokesman for Spain's Foreign Ministry told CNN.

Earlier, the mayor of the Spanish town of Betanzos, Ramon Garcia Vasquez, had told CNN that Frade was a Spanish citizen. Frade's family and relatives live in Betanzos, according to Vasquez.

Aysha Frade was also named as a victim of the terror attack in central London.

US citizen Kurt Cochran was in London celebrating his 25th wedding anniversary with his wife Melissa, who suffered a broken leg, broken rib and cut to her head, her brother Clint Payne posted to social media.

"Our hearts are broken this day as we say good-bye to our dear brother-in-law Kurt. He passed away from the injuries sustained in the London terrorist attack yesterday," he posted.

US President Donald Trump tweeted: "A great American, Kurt Cochran, was killed in the London terror attack. My prayers and condolences are with his family and friends."

Kurt Cochran (left) was in London celebrating his 25th wedding anniversary with his wife Melissa (right).

The unidentified fourth victim, a 75-year-old man, died Thursday night after his life support was withdrawn at a hospital where he had been receiving treatment, according to Met Police.

A number of tourists were among 40 people hurt in the assault, including five South Koreans and three French high school students. One Australian was hospitalized. A Chinese foreign ministry spokeswoman said one Chinese tourist was slightly injured.

May also listed citizens of Romania, Germany, Poland, Ireland, Italy and Greece among the victims. Twelve Britons required hospital treatment.

UK Foreign Secretary Boris Johnson earlier led the UN Security Council in a moment of silence for the victims of terror in London and elsewhere.

"There are victims in London from 11 nations, which goes to show that an attack on London is an attack on the world," he said.

"And I can tell you from my talks here in the United States, with the US government and with partners from around the world, that the world is united to defeat the people who launched this attack, and to defeat their bankrupt and odious ideology."

The image of Britain's Union Flag was projected on the Brandenburg Gate in Berlin Thursday night in solidarity with London, while the Eiffel Tower went dark Wednesday in Paris, a city shaken by several attacks in recent years.